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Correct explanation for change in pH?


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#1 Violagirl

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Posted 31 August 2013 - 07:49 PM

I'm working on a prelab for my Biochem lab course and one question I had was:

If Hepes is given in basic form and if you need to make a buffer of pH 8, will you have to add HCl or NaOH? Why?

I stated that because Hepes has a pKa of 7.55, that you would need to add NaOH in order to raise the pH level to 8 so that H ions will dissociate from the acidic form of Hepes and form more of the conjugate base form of Hepes. 

Is this the correct way to think about this question? Thanks for reading!



#2 mdfenko

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Posted 01 September 2013 - 02:54 PM

no.

here is a review of dissociation constant on wikipedia.

 

buffers resist change towards the pK, it is not the pH that you get when you dissolve the buffer compound.

 

you are told that you are dissolving the basic form of hepes. you will have to reduce the pH to 8.


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